Study On Dielectric Recovery Characteristics After Non-equilibrium Plasma Arc In SF₆ Circuit Breaker

SF₆ circuit breaker is the key device for large capacity and long distance power transmission,development of smart grid and new energy resources in power system.Stricter requests have been proposed for the breaking capacity and reliability of SF₆ circuit breaker,with the installation of smart ultra high-voltage?UHV?power system and the development of global energy interconnection.Recently,breaking failures caused by the re-strike happened frequently,along with the increasing of voltage level and transmission capacity,especially for the capacitor bank breaker in UHV project used to break arc with small current and short arcing time.In above situation,the transient recovery voltage is higher and electric field intensity is larger due to short clearance between arcing contacts,therefore re-strike is easy to happen and the electrical lifetime of circuit breaker is badly limited.So several research directions become of utmost urgency,like the dielectric recovery characteristics after arcing and the influences of gas flow on SF₆ insulation strength in different breaking methods.Then the work about optimizing the breaking method and improving the breaking capacity and electrical lifetime could be executed.On the foundation of research about collision process between particles in SF₆ plasma,the re-strike mechanism of hot SF₆ was revealed and the numerical calculating method for whole breaking process in SF₆ circuit breaker was proposed.The dielectric recovery characteristics and its influencing parameters were studied as well.Related researches are summarized as follows:?1?The compositions and contents of non-equilibrium SF₆ plasma were calculated based on method of mass action law.Then by solving the Fokker-Planck dynamic equation which describing the collision process of charged particles,the slowing down frequency,deflection frequency,parallel diffusion frequency and energy exchange frequency of electrons were obtained.Though solving the energy change rate between particles,time to reach temperature balance between electrons,heavy particles,electrons and heavy particles due to collision were settled.The mass density,specific enthalpy and specific heat at constant were computed from employing standard thermodynamic relationships,and then the influence of non-equilibrium coefficients on thermodynamic parameters was analyzed.The diffusion coefficients,viscosity,thermal conductivity and electrical conductivity were computed through utilizing the Chapman-Enskog the method,and then the influence of non-equilibrium coefficients on transport coefficient was analyzed.A database was established to describe the relationship between micro-collision process in SF₆ plasma and macro-properties of arc,with pressure of0.1MPa-2.0MPa and temperature of 300K-40000K.?2?A two-term approximate method was applied into solution of Boltzmann equation to obtain the electron energy distribution function of hot SF₆.Then the ionization and adsorption reaction coefficient of every collision process were calculated under different reduced electric field intensities.Finally,according to the Pedersen criterion,the total reduced ionization and adsorption coefficient were derived,which can be used to reveal the re-strike mechanism in hot SF₆.Based on above work,a database about reduced breakdown electrical field intensity of hot SF₆ under pressure of 0.1MPa-2.0MPa and temperature of 3500K-300K.Research results of this part are important theoretical standard to estimate re-strike of hot SF₆ after arc.?3?A two-temperature model about non-equilibrium SF₆ arc plasma was built according to mass conservation equation,momentum conservation equation,energy conservation equation and gas equation of state,and the influence of turbulence on gas flow was taken into account as well,thus forming a numerical computing method for the whole breaking process in LW35C-126 SF₆ circuit breaker.The electrical field distribution and field nonuniformity coefficient in arc extinguish chamber were computed through electrical field analysis,while the spatial distribution dynamic characteristics of several parameters like pressure,mass density,electron temperature,heavy particle temperature,nonequilibrium coefficient and particle number density were analyzed through the flow field in condition of no-load and breaking 1600A capacitive current.Results show that the SF₆ at terminal of upper arc contact owns large flow velocity,small density and low breakdown voltage when the arcing contacts are just separated,the upper arc contact moves to the throat of the small nozzle,and upper arc contact just saperates with the throat of big nozzle,thus causing these locations with relatively weak dielectric strength in breaking process.?4?To prove this numerical computation method that calculates SF₆ breaking voltage in the whole breaking process in circuit breaker,a testing circuit of dynamic breaking voltage of between contacts in circuit breaker was executed.The testing was operated under pressure of0.5MPa-0.8MPa,taking the SF₆ gas flow influence into consideration.By combining the testing and computing results without load,this method was proved,and the dielectric recovery characteristics of cold SF₆ in circuit breaker for capacitor bank was analyzed meanwhile.The dielectric recovery characteristics of hot SF₆ were analyzed with condition of breaking 1600A capacity current and short arcing time.Influences of breaking phase angel,velocity and gas pressure were discussed for LW35C-126 SF₆ circuit breaker at the same time.After taking all sides into consideration,such as insulation margin,burning and erosion after long time arcing,and mechanical dispersion of 0.1?,the breaking phase angle of 0.5?was proposed.In the premise of improving the operating devices power not too much and guarantee of reliability,the breaking velocity could be improved in a proper range,but method of reducing the gas pressure was not recommended.